1. Field of the Invention
The present invention concerns a mammography apparatus in which x-ray sources have different distances from the chest.
2. Description of the Prior Art
In a tomosynthesis procedure, a three-dimensional image is generated from a number of two-dimensional images. A two-dimensional image or a projection of the tissue to be examined that the x-ray beam passes through is generated by an x-ray device with an x-ray source and a detector. The two-dimensional image depicts the attenuation of the tissue in the volume that the beam has passed through. A second two-dimensional image or a second projection of the same tissue or volume is acquired after the radiation source and/or the detector has been moved into a second position. A three-dimensional tomosynthesis image can be generated by a computerized reconstruction after a number of such two-dimensional images have been acquired.
One field of application of the aforementioned three-dimensional imaging method is mammography. An image generation device that is typically used in mammography has a pivotable x-ray source and a stationary x-ray detector. The tissue to be examined is positioned over the stationary detector. The x-ray source is subsequently pivoted in multiple steps, for example in a range of +/−25°, and a number of two-dimensional x-ray images is acquired from different pivot positions of the x-ray source with the stationary detector. Naturally, it is also possible to use a number of stationary x-ray sources or to displace the x-ray source only in a translational manner. The detector can also be shifted or pivoted counter to the movement of the x-ray source. The x-ray source(s) emits or emit x-rays from positions that are arranged along a line that runs parallel to the shoulder-to-shoulder axis of a patient. By using a beam path parallel to the chest wall, the entire tissue of the breast is imaged and the chest is not exposed to radiation. A three-dimensional image is generated from the multiple two-dimensional x-ray images by means of the reconstruction. Imaging methods and devices for the mammography of this type are described in DE 10 2006 046 741 A1, DE 10 2008 004 473 A1, DE 10 2008 033 150 A1, EP 2 138 098 A1 and DE 10 2008 028 387 A1, for example.
Filtered back projection techniques (that are described Imaging Systems for Medical Diagnostics, Arnulf Oppelt, Publicis Corporate Publishing, Erlangen, ISBN 3-89578-226-2, Chapter 10.5, for example) are known for use for the reconstruction of a three-dimensional image from a plurality of two-dimensional images. These filtered back projection methods present reconstructed images with a high contrast and a high accuracy of detail but lose information about the relative tissue density due to the missing data given tomosynthesis with limited scan angle.
The breast is composed primarily of glandular tissue, fat tissue, connective tissue and blood vessels. The x-ray attenuation coefficients of these tissue types are very similar, which significantly hinders the evaluation of three-dimensional mammography images. The primary field of application of imaging methods in mammography is the early detection of cancerous tissue. It is additionally made more difficult in that cancerous tissue has an x-ray attenuation coefficient similar to other tissue types. Mammography methods are described in Imaging Systems for Medical Diagnostics, Arnulf Oppelt, Chapter 12.6, Publicis Corporate Publishing, Erlangen, ISBN 3-89578-226-2, for example.
As mentioned above, in the prior art an x-ray source is moved, or x-ray sources are moved along a line that runs parallel to the shoulder-to-shoulder axis of the patient. The linear scanning over a limited angle range leads to a limited depth resolution. Therefore, conventional mammography apparatuses and mammography methods have a limited sensitivity and specificity.